Chronometric speedos

[will_curry]

Over the years I've acquired a number of distressed chronometric speedos. All of them have
identification marks like 'S433/1/L' and most have numbers as well such as 1450, 1550 and 1695.
These I believe represent the number of turns the cable makes in travelling one mile.

I was hoping to build one decent speedo from the various carcasses. Naturally, there are questions:

- So, apart from the dial, what differentiates between 80mph and 120mph speedos?
- Similarly, how are the different cable turns per mile arranged?